In surface drilling, a drill bit at one end of a drill rod is suspended from a traveling block in a drilling derrick and used to crush and cut a bore hole in the earth. The drilling derrick may be as much as 150 feet high and designed for loads of up to 600 tons. If a drilling operation is on a more limited scale, a collapsible mast on a mobile chassis may be used to suspend the drill string.
A square rod or kelly, suspended from the kelly bushing, engages a socket in a power-driven turntable which transmits rotary motion to the drill string and thus to the drill bit attached to the end of the string. The drill rod and extensions are typically hollow. During drilling, a fluid is pumped down through the rod which rises to the surface through the annular space between the rod and the wall of the bore hole. The liquid, which in most instances consists of water to which certain substances are added to increase its specific gravity, is referred to as "drilling mud" and is kept in circulation in the bore hole by a pump. The drilling mud cools the drill bit and removes cuttings from the bottom of the drill hole by carrying them to the surface in the annular space between the drill string and the bore hole wall.
On emerging at the surface, the drilling mud is passed through a vibrating screen or "shale shaker" to remove the course debris. The remainder of the drilling mud is deposited in a tank where the sand and silt-sized particles are removed by a series of cyclonic separators; one set to remove the sand-sized particles, and another set for the silt-sized particles. The drilling mud is then moved to a settling tank before being circulated to a suction pit and pumped back down the bore hole.
At one time, the principal surface feature of the drilling mud circulating system was merely a pit, called a reserve pit, dug in the ground adjacent the well. The reserve pit had little to do with fluid circulation, except for the special occasion when the drilling mud was circulated through the reserve pit. Reserve pits were mostly used as a depository for waste fluid and cuttings. In many instances, the only provision for fluid transfer from the reserve pit to the active system was by a small pump. However, because of ever-increasing environmental concerns, many state agencies have banned the use of in-ground reserve pits.
Today, in place of in-ground reserve pits, extra tanks are being used. The drilling fluid is slowly circulated through the tanks to allow solids to settle out. The sediment is then scooped or flushed from the tanks and loaded on trucks so the debris may be deposited in a special landfill. Because the drilling mud is circulated through the tanks, water supplies are better defined and closely calculated.
Recent drilling mud circulation systems have used centrifuge separators to remove the solids while retrieving a substantial portion of the water. A problem with such systems is their inability to adjust to irregular densities of solids carried by the drilling mud. A large influx of solids into the centrifuge separators produces a sharp increase in torque on the system, sometimes resulting in a shearing of the drive mechanism. When this occurs, the entire drilling process must stop until the circulation system is brought back on-line.
There has been a long-felt, yet unresolved need, to adjust to varying densities to avoid extreme torque which could damage the separators. Moreover, a need also exists for a system wherein any one of a number of compartments or tanks may selectively receive and temporarily store untreated or treated drilling mud to be used in the drilling operation in order to provide a continuous and sufficient supply of clean drilling mud. Lastly, there has been a long-felt, yet unresolved need, for a system to remove cuttings from the drilling mud wherein the cuttings may be disposed in landfills without the need for special permits since the cuttings are substantially free of hazardous or undesirable contaminants.